1. Field of the Invention
The present invention relates to a photoelectric conversion element equipped with a lower electrode, an upper electrode facing the lower electrode, and an organic photoelectric conversion layer formed between the lower electrode and the upper electrode, and to a solid-state imaging device.
2. Description of Related Art
With organic thin film solar cells, their performance is evaluated without applying thereto an external electric field, since their purpose is to produce electric power from them. However, with organic photoelectric conversion elements which are required to provide maximum photoelectric conversion efficiency, such as an image input element or a photo-sensor, an external voltage is in many cases applied thereto for improving photoelectric conversion efficiency and response speed. In such cases, dark current increases due to hole injection or electron injection from an electrode by application of the external electric field, thus an element having a large photo current/dark current ratio not having been obtained. Therefore, a technology of suppressing dark current without reducing photo current can be said to be one of the key technologies of the photoelectric conversion element.
It has been known to suppress dark current by inserting between an electrode and a photoelectric conversion layer a blocking layer which functions as a Schottky barrier for carrier injection (dark current) from the electrode. With this method, in order to suppress carrier injection from the electrode upon application of an external electric field, a material capable of enlarging the Schottky barrier against the electrode as much as possible is used for the blocking layer. As the material for the blocking layer, silicon dioxide, silicon monoxide, and the like are known. As known photoelectric conversion elements equipped with the blocking layer, there are illustrated, for example, those described in JP-A-59-229860 and JP-A-5-129576.
Incidentally, use of silicon dioxide as a material for a blocking layer provides enough high insulating properties to suppress dark current, but has involved the problem of reduction in photoelectric conversion efficiency since silicon dioxide inhibits carriers generated by photoelectric conversion as well. Also, in the case of using silicon monoxide, it provides a smaller effect of suppressing dark current due to its higher electrical conductivity than silicon dioxide, though the reduction in photoelectric conversion is small, thus having failed to provide a sufficient S/N ratio.
In order to solve such problems, there has been known a method of inserting an insulating layer as a blocking layer between the electrode and the photoelectric conversion layer to thereby suppress dark current by carrier injection from the electrode.
For example, in JP-A-59-229860, it is described that dark current can be reduced by disposing an insulating film comprising silicon dioxide (partly containing silicon monoxide) between the electrode and the photoelectric conversion layer through a sputtering method or through oxidation of silicon. However, though dark current is truly suppressed by increasing the thickness of the insulating film, the thick insulating layer inhibits read-out of signal carriers as well, thus there results reduced photoelectric conversion efficiency. On the other hand, when the thickness of the insulating film is small, sufficient effect of suppressing dark current is unable to be obtained though reduction in the photoelectric conversion efficiency is lowered. In either case, sufficient S/N ratio is not obtained. Also, in the case of using a highly insulating film, there has involved another problem that there occurs response speed delay. As the reason for this, accumulation of signal carriers at the interface of the insulating layer or in the insulating layer can be considered, but sufficient countermeasures have not been proposed.
Also, in JP-A-5-129576, a comparatively rapid response speed can be obtained with suppressing reduction in the photoelectric conversion efficiency, by using silicon monoxide having a comparatively high electric conductivity but, since insulating properties of silicon monoxide are less sufficient in comparison with silicon dioxide or the like, high S/N ratio is unable to be obtained.